Our solar system consists of an average star we call the
Sun, the planets
Mercury, Venus,
Earth, Mars,
Jupiter, Saturn,
Uranus, Neptune, and
Pluto. It includes: the satellites of the
planets; numerous comets,
asteroids, and meteoroids; and the
interplanetary medium. The Sun is the richest source of electromagnetic
energy (mostly in the form of heat and light) in the solar system. The
Sun's nearest known stellar neighbor is a red dwarf star called Proxima
Centauri, at a distance of 4.3 light
years away. The whole solar system, together with the local stars
visible on a clear night, orbits the center of our home galaxy, a spiral
disk of 200 billion stars we call the Milky Way.
The Milky Way has two small galaxies orbiting it nearby, which are
visible from the southern hemisphere. They are called the Large
Magellanic Cloud and the Small Magellanic Cloud. The nearest large
galaxy is the Andromeda Galaxy. It is a spiral
galaxy like the Milky Way but is 4 times as massive and is 2 million
light years away. Our galaxy, one of billions of galaxies known, is
traveling through intergalactic space.

The planets, most of the satellites of the planets and the
asteroids revolve around the Sun in the same direction,
in nearly circular orbits.
When looking down from above the Sun's north pole,
the planets orbit in a counter-clockwise direction.
The planets orbit the Sun in or near the same plane, called the
ecliptic. Pluto is a special
case in that its orbit is the most
highly inclined (18 degrees) and the most highly elliptical of all the
planets. Because of this, for part of its orbit, Pluto is closer
to the Sun than is
Neptune. The axis of rotation for most of the planets
is nearly perpendicular to the ecliptic. The exceptions are
Uranus and Pluto,
which are tipped on their sides.

The Sun contains 99.85% of all the matter in the Solar System.
The planets, which condensed out of the same disk of material
that formed the Sun, contain only 0.135% of the mass of the solar system.
Jupiter contains more than twice the matter of all the other planets
combined. Satellites of the planets, comets, asteroids, meteoroids,
and the interplanetary medium constitute the remaining 0.015%.
The following table is a list of the mass distribution within
our Solar System.

Nearly all the solar system by volume appears to be an empty void.
Far from being nothingness, this vacuum of "space" comprises the
interplanetary medium. It includes various forms of energy and at
least two material components: interplanetary dust and interplanetary
gas. Interplanetary dust consists of microscopic solid particles.
Interplanetary gas is a tenuous flow of gas and charged particles,
mostly protons and electrons -- plasma
-- which stream from the Sun, called the
solar wind.

The solar wind can be measured by spacecraft, and it has a large effect
on comet tails. It also has a measurable effect on the motion of
spacecraft. The speed of the solar wind is about 400 kilometers
(250 miles) per second
in the vicinity of Earth's orbit. The point at which the solar wind
meets the interstellar medium, which is the "solar" wind from other
stars, is called the heliopause. It is a boundary theorized to be
roughly circular or teardrop-shaped, marking the edge of the Sun's
influence perhaps 100 AU from the Sun. The space within the boundary
of the heliopause, containing the Sun and solar system, is referred to
as the heliosphere.

The solar magnetic field extends outward into interplanetary space;
it can be measured on Earth and by spacecraft. The solar magnetic
field is the dominating magnetic field throughout the
interplanetary regions of the solar system, except in the immediate
environment of planets which have their own magnetic fields.

The terrestrial planets are the four innermost planets in the solar
system, Mercury,
Venus, Earth and
Mars. They are called terrestrial because they
have a compact, rocky surface like the Earth's. The planets, Venus,
Earth, and Mars have significant atmospheres while Mercury has almost
none. The following diagram shows the approximate distance of the
terrestrial planets to the Sun.

Jupiter, Saturn,
Uranus, and Neptune
are known as the Jovian (Jupiter-like) planets, because they are all
gigantic compared with Earth, and they have a gaseous nature like
Jupiter's. The Jovian planets are also referred to as the gas
giants, although some or all of them might have small solid cores.
The following diagram shows the approximate distance of the Jovian
planets to the Sun.

Our Milkyway Galaxy
This image of our galaxy, the Milky Way, was taken with NASA's Cosmic
Background Explorer's (COBE) Diffuse Infrared Background Experiment
(DIRBE). This never-before-seen view shows the Milky Way from an
edge-on perspective with the galactic north pole at the top, the south pole
at the bottom and the galactic center at the center. The picture combines
images obtained at several near-infrared wavelengths. Stars within our
galaxy are the dominant source of light at these wavelengths. Even
though our solar system is part of the Milky Way, the view looks distant
because most of the light comes from the population of stars that are
closer to the galactic center than our own Sun.
(Courtesy NASA)

Our Milky Way Gets a Makeover
Like early explorers mapping the continents of our globe, astronomers are
busy charting the spiral structure of our galaxy, the Milky Way. Using
infrared images from NASA's Spitzer Space Telescope, scientists have
discovered that the Milky Way's elegant spiral structure is dominated by
just two arms wrapping off the ends of a central bar of stars. Previously,
our galaxy was thought to possess four major arms.

This artist's concept illustrates the new view of the Milky Way, along
with other findings presented at the 212th American Astronomical Society
meeting in St. Louis, Mo. The galaxy's two major arms (Scutum-Centaurus
and Perseus) can be seen attached to the ends of a thick central bar,
while the two now-demoted minor arms (Norma and Sagittarius) are less
distinct and located between the major arms. The major arms consist of the
highest densities of both young and old stars; the minor arms are
primarily filled with gas and pockets of star-forming activity.

The artist's concept also includes a new spiral arm, called the "Far-3
kiloparsec arm," discovered via a radio-telescope survey of gas in the
Milky Way. This arm is shorter than the two major arms and lies along the
bar of the galaxy.

Our sun lies near a small, partial arm called the Orion Arm, or Orion
Spur, located between the Sagittarius and Perseus arms.
(Courtesy NASA/JPL-Caltech)

Spiral Galaxy, NGC 4414
The majestic galaxy, NGC 4414, is located 60 million light-years away.
Like the Milky Way, NGC 4414 is a giant spiral-shaped disk of stars, with a bulbous
central hub of older yellow and red stars.
The outer spiral arms are considerably bluer due to ongoing
formation of young, blue stars, the brightest of which can be seen
individually at the high resolution provided by the Hubble camera.
The arms are also very rich in clouds of interstellar dust, seen as
dark patches and streaks silhouetted against the starlight.
(Courtesy NASA/STSCI)

Obliquity of the Eight Planets
This illustration shows the obliquity of the eight planets. Obliquity
is the angle between a planet's equatorial plane and its orbital plane.
By International Astronomical Union (IAU) convention, a planet's north
pole lies above the ecliptic plane. By this convention, Venus, Uranus,
and Pluto have a retrograde rotation, or a rotation that is in the opposite
direction from the other planets.
(Copyright 2008 by Calvin J. Hamilton)

The Solar System
During the past three decades a myriad of space explorers
have escaped the confines of planet Earth and have set out
to discover our planetary neighbors. This picture shows
the Sun and all nine planets of the solar system as seen
by the space explorers. Starting at the top-left corner
is the Sun followed by the planets
Mercury, Venus,
Earth, Mars,
Jupiter, Saturn,
Uranus, Neptune,
and Pluto.
(Copyright 1998 by Calvin J. Hamilton)

Jovian Planets
This image shows the Jovian planets Jupiter,
Saturn, Uranus
and Neptune approximately to scale.
The Jovian planets are named because of their gigantic Jupiter-like
appearance.
(Copyright Calvin J. Hamilton)

The Largest Moons and Smallest Planets
This image shows the relative sizes of the largest moons and the
smallest planets in the solarsystem. The largest satellites pictured in this image are:
Ganymede (5262 km),
Titan (5150 km),
Callisto (4806 km),
Io (3642 km),
the Moon (3476 km),
Europa (3138 km),
Triton (2706 km),
and Titania (1580 km).
Both Ganymede and Titan are larger than planet
Mercury followed
by Io, the Moon, Europa, and Triton which are larger than
the planet Pluto.
(Copyright Calvin J. Hamilton)

Diagram of Portrait Frames
On February 14, 1990, the cameras of Voyager 1
pointed back toward the Sun and took a
series of pictures of the Sun and the planets, making the first ever
"portrait" of our solar system as seen from the outside.
This image is a diagram of how the frames for the solar system portrait
were taken.
(Courtesy NASA/JPL)

All Frames from the Family Portrait
This image shows the series of pictures of the Sun and the
planets taken on February 14, 1990,
for the solar system family portrait
as seen from the outside. In the course of taking this mosaic
consisting of a total of 60 frames, Voyager 1
made several images of the inner solar system from a distance of
approximately 6.4 billion kilometers (4 billion miles) and about 32° above
the
ecliptic plane. Thirty-nine wide angle
frames link together six of the planets of our solar system in this
mosaic. Outermost Neptune is 30 times
further from the Sun than Earth. Our
Sun is seen as the bright object in the center
of the circle of frames. The insets show the planets magnified many
times.
(Courtesy NASA/JPL)

Portrait of the Solar System
These six narrow-angle color images were made from the first ever
"portrait" of the solar system taken by
Voyager 1, which was more than
6.4 billion kilometers (4 billion miles)
from Earth and about 32°
above the ecliptic.
Mercury
is too close to the Sun to be seen.
Mars was not detectable by the Voyager cameras
due to scattered sunlight in the optics, and
Pluto was not included in the mosaic because of
its small size and distance from the Sun. These blown-up images, left to
right and top to bottom are Venus,
Earth, Jupiter,
Saturn, Uranus, and
Neptune.
(Courtesy NASA/JPL)

* The Sun's period of rotation at the surface varies from
approximately 25 days at the equator to 36 days at the poles. Deep
down, below the convective
zone, everything appears to rotate with
a period of 27 days.